Method for frequency division duplex communications

ABSTRACT

The high quality PCS communications are enabled in environments where adjacent PCS service bands operate with out-of-band harmonics that would otherwise interfere with the system&#39;s operation. The highly bandwidth-efficient communications method combines a form of time division duplex (TDD), frequency division duplex (FDD), time division multiple access (TDMA), orthogonal frequency division multiplexing (OFDM), spatial diversity, and polarization diversity in various unique combinations. The method provides excellent fade resistance. The method enables changing a user&#39;s available bandwidth on demand by assigning additional TDMA slots during the user&#39;s session.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.13/116,375, filed May 26, 2011, which is a continuation of U.S. patentapplication Ser. No. 12/192,705, filed Aug. 15, 2008, (U.S. Pat. No.7,983,217), which is a continuation of U.S. patent application Ser. No.11/022,048, filed Dec. 22, 2004, (U.S. Pat. No. 7,450,542), which is acontinuation of U.S. patent application Ser. No. 10/383,976, filed Mar.7, 2003 (U.S. Pat. No. 6,853,629), which is a continuation of U.S.patent application Ser. No. 09/294,174, filed Apr. 19, 1999 (U.S. Pat.No. 6,560,209), which is a continuation of U.S. patent application Ser.No. 08/796,584, filed Feb. 6, 1997 (U.S. Pat. No. 5,933,421), all ofwhich are hereby incorporated by reference in their entirety.

The invention disclosed herein is related to U.S. patent application byGibbons, et al entitled “REMOTE WIRELESS UNIT HAVING REDUCED POWEROPERATING MODE,” Ser. No. 08/796,586, filed on Feb. 6, 1997 (U.S. Pat.No. 6,085,114), which application is incorporated herein by reference.

The invention disclosed herein is related to the copending U.S. patentapplication by Greg Veintimilla, entitled “METHOD TO INDICATESYNCHRONIZATION LOCK OF A REMOTE STATION WITH A BASE STATION,” Ser. No.08/796,492, filed on Feb. 6, 1997 (U.S. Pat. No. 5,943,375), whichapplication is incorporated herein by reference.

The invention disclosed herein is related to the copending U.S. patentapplication by Elliott Hoole, entitled “DELAY COMPENSATION,” Ser. No.08/796,491, filed on Feb. 6, 1997 (U.S. Pat. No. 5,799,000), whichapplication is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention involves improvements to communications systems andmethods in a wireless, frequency division duplex communications system.

2. Description of the Related Art

Wireless communications systems, such as cellular and personalcommunications systems, operate over limited spectral bandwidths. Theymust make highly efficient use of the scarce bandwidth resource toprovide good service to a large population of users. Examples of suchcommunications systems that deal with high user demand and scarcebandwidth resources are wireless communications systems, such ascellular and personal communications systems.

Various techniques have been suggested for such systems to increasebandwidth-efficiency, the amount of information that can be transmittedwithin a given spectral bandwidth. Many of these techniques involvereusing the same communication resources for multiple users whilemaintaining the identity of each user's message. These techniques aregenerically referred to as multiple access protocols. Among thesemultiple access protocols are Time Division Multiple Access (TDMA), CodeDivision Multiple Access (CDMA), Space Division Multiple Access (SDMA),and Frequency Division Multiple Access (FDMA). The technical foundationsof these multiple access protocols are discussed in the recent book byRappaport entitled “Wireless Communications Principles and Practice,”Prentice Hall, 1996.

The Time Division Multiple Access (TDMA) protocol sends information froma multiplicity of users on one assigned frequency bandwidth by timedivision multiplexing the information from the various users. In thismultiplexing scheme, particular time slots are devoted to specificusers. Knowledge of the time slot during which any specific informationis transmitted, permits the separation and reconstruction of each user'smessage at the receiving end of the communication channel.

The Code Division Multiple Access (CDMA) protocol uses a unique code todistinguish each user's data signal from other users' data signals.Knowledge of the unique code with which any specific information istransmitted, permits the separation and reconstruction of each user'smessage at the receiving end of the communication channel. There arefour types of CDMA protocols classified by modulation: direct sequence(or pseudo-noise), frequency hopping, time hopping, and hybrid systems.The technical foundations for CDMA protocols are discussed in the recentbook by Prasad entitled “CDMA for Wireless Personal Communications,”Artech House, 1996.

The Direct Sequence CDMA (DS-CDMA) protocol spreads a user's data signalover a wide portion of the frequency spectrum by modulating the datasignal with a unique code signal that is of higher bandwidth than thedata signal. The frequency of the code signal is chosen to be muchlarger than the frequency of the data signal. The data signal isdirectly modulated by the by the code signal and the resulting encodeddata signal modulates a single, wideband carrier that continuouslycovers a wide frequency range. After transmission of the DS-CDMAmodulated carrier signal, the receiver uses a locally generated versionof the user's unique code signal to demodulate the received signal andobtain a reconstructed data signal. The receiver is thus able to extractthe user's data signal from a modulated carrier that bears many otherusers' data signals.

The Frequency Hopping Spread Spectrum (FHSS) protocol uses a unique codeto change a value of the narrowband carrier frequency for successivebursts of the user's data signal. The value of the carrier frequencyvaries in time over a wide range of the frequency spectrum in accordancewith the unique code. The term Spread Spectrum Multiple Access (SSMA) isalso used for CDMA protocols such as DS-CDMA and FHSS that use arelatively wide frequency range over which to distribute a relativelynarrowband data signal.

The Time Hopping CDMA (TH-CDMA) protocol uses a single, narrowbandwidth, carrier frequency to send bursts of the user's data atintervals determined by the user's unique code. Hybrid CDMA systemsinclude all CDMA systems that employ a combination of two or more CDMAprotocols, such as direct sequence/frequency hopping (DS/FH), directsequence/time hopping (DS/TH), frequency hopping/time hopping (FH/TH),and direct sequence/frequency hopping/time hopping (DS/FH/TH).

The Space Division Multiple Access (SDMA) transmission protocol formsdirected beams of energy whose radiation patterns do not overlapspatially with each other, to communicate with users at differentlocations. Adaptive antenna arrays can be driven in phased patterns tosimultaneously steer energy in the direction of selected receivers. Withsuch a transmission technique, the other multiplexing schemes can bereused in each of the separately directed beams. For example, thespecific codes used in CDMA can be transmitted in two different beams.Accordingly, if the beams do not overlap each other, different users canbe assigned the same code as long as they do not receive the same beam.

The Frequency Division Multiple Access (FDMA) protocol services amultiplicity of users over one frequency band by devoting particularfrequency slots to specific users, i.e., by frequency divisionmultiplexing the information associated with different users. Knowledgeof the frequency slot in which any specific information resides permitsreconstruction of each user's information at the receiving end of thecommunication channel.

Orthogonal Frequency Division Multiplexing (OFDM) addresses a problemthat is faced, for example, when pulsed signals are transmitted in anFDMA format. In accordance with principles well known in thecommunication sciences, the limited time duration of such signalsinherently broadens the bandwidth of the signal in frequency space.Accordingly, different frequency channels may significantly overlap,defeating the use of frequency as a user-identifying-parameter, theprinciple upon which FDMA is based. However, pulsed information that istransmitted on specific frequencies can be separated, in accordance withOFDM principles, despite the fact that the frequency channels overlapdue to the limited time duration of the signals. OFDM requires aspecific relationship between the data rate and the carrier frequencies.Specifically, the total signal frequency band is divided into Nfrequency sub-channels, each of which has the same data rate 1/T. Thesedata streams are then multiplexed onto a multiplicity of carriers thatare separated in frequency by 1/T. Multiplexing signals under theseconstraints results in each carrier having a frequency response that haszeroes at multiples of 1/T. Therefore, there is no interference betweenthe various carrier channels, despite the fact that the channels overlapeach other because of the broadening associated with the data rate. OFDMis disclosed, for example, by Chang in Bell Sys. Tech. Jour., Vol. 45,pp. 1775-1796, December 1966, and in U.S. Pat. No. 4,488,445.

Parallel Data Transmission is a technique related to FDMA. It is alsoreferred to as Multitone Transmission (MT), Discrete MultitoneTransmission (DMT) or Multi-Carrier Transmission (MCT). Parallel DataTransmission has significant calculational advantages over simple FDMA.In this technique, each user's information is divided and transmittedover different frequencies, or “tones,” rather than over a singlefrequency, as in standard FDMA. In an example of this technique, inputdata at NF bits per second are grouped into blocks of N bits at a datarate of F bits per second. N carriers or “tones” are then used totransmit these bits, each carrier transmitting F bits per second. Thecarriers can be spaced in accordance with the principles of OFDM.

Both the phase and the amplitude of the carrier can be varied torepresent the signal in multitone transmission. Accordingly, multitonetransmission can be implemented with M-ary digital modulation schemes.In an M-ary modulation scheme, two or more bits are grouped together toform symbols and one of the M possible signals is transmitted duringeach symbol period. Examples of M-ary digital modulation schemes includePhase Shift Keying (PSK), Frequency Shift Keying (FSK), and higher orderQuadrature Amplitude Modulation (QAM). In QAM a signal is represented bythe phase and amplitude of a carrier wave. In high order QAM, amultitude of points can be distinguished on an amplitude/phase plot. Forexample, in 64-ary QAM, 64 such points can be distinguished. Since sixbits of zeros and ones can take on 64 different combinations, a six-bitsequence of data symbols can, for example, be modulated onto a carrierin 64-ary QAM by transmitting only one value set of phase and amplitude,out of the possible 64 such sets.

Suggestions have been made to combine some of the above temporal andspectral multiplexing techniques. For example, in U.S. Pat. No.5,260,967, issued to Schilling, there is disclosed the combination ofTDMA and CDMA. In U.S. Pat. No. 5,291,475, issued to Bruckert, and inU.S. Pat. No. 5,319,634 issued to Bartholomew, the combination of TDMA,FDMA, and CDMA is suggested.

Other suggestions have been made to combine various temporal andspectral multiple-access techniques with spatial multiple-accesstechniques. For example, in U.S. Pat. No. 5,515,378, filed Dec. 12,1991, Roy suggests “separating multiple messages in the same frequency,code, or time channel using the fact that they are in different spatialchannels.” Roy suggests specific application of his technique to mobilecellular communications using an “antenna array.” Similar suggestionswere made by Swales et al., in the IEEE Trans. Veh. Technol. Vol. 39,No. 1, February 1990, and by Davies et al. in A.T.R., Vol. 22, No. 1,1988 and in Telecom Australia, Rev. Act., 1985/86 pp. 41-43.

Gardner and Schell suggest the use of communications channels that are“spectrally disjoint” in conjunction with “spatially separable”radiation patterns in U.S. Pat. No. 5,260,968, filed Jun. 23, 1992. Theradiation patterns are determined by restoring “self coherence”properties of the signal using an adaptive antenna array. “[A]n adaptiveantenna array at a base station is used in conjunction with signalprocessing through self coherence restoral to separate the temporallyand spectrally overlapping signals of users that arrive from differentspecific locations.” See the Abstract of the Invention. In this patent,however, adaptive analysis and self coherence restoral is only used todetermine the optimal beam pattern; “ . . . conventional spectralfilters . . . [are used] . . . to separate spatially inseparablefilters.”

Winters suggests “adaptive array processing” in which “[t]he frequencydomain data from a plurality of antennas are . . . combined for channelseparation and conversion to the time domain for demodulation,” in U.S.Pat. No. 5,481,570, filed Oct. 20, 1993. Column 1, lines 66-67 andColumn 2, lines 14-16.

Agee has shown that “the use of an M-element multiport antenna array atthe base station of any communication network can increase the frequencyreuse of the network by a factor of M and greatly broaden the range ofinput SINRs required for adequate demodulation . . . ” (“WirelessPersonal Communications: Trends and Challenges,” Rappaport, Woerner andReed, editors, Kluwer Academic Publishers, 1994, pp. 69-80, at page 69.See also, Proc. Virginia Tech. Third Symposium on Wireless PersonalCommunications, June 1993, pp. 15-1 to 15-12.)

Gardner and Schell also suggest in U.S. Pat. No. 5,260,968, filed Jun.23, 1992, “time division multiplexing of the signal from the basestation and the users” . . . “[i]n order to use the same frequency forduplex communications . . . ” “[R]eception at the base station from allmobile units is temporally separated from transmission from the basestation to all mobile units.” Column 5, lines 44ff. In a similar vein,in U.S. Pat. No. 4,383,332 there is disclosed a wireless multi-elementadaptive antenna array SDMA system where all the required adaptivesignal processing is performed at baseband at the base station throughthe use of “time division retransmission techniques.”

Fazel, “Narrow-Band Interference Rejection in Orthogonal Multi-CarrierSpread-Spectrum Communications,” Record, 1994 Third Annual InternationalConference on Universal Personal Communications, IEEE, 1994, pp. 46-50describes a transmission scheme based on combined spread spectrum andOFDM. A plurality of subcarrier frequencies have components of thespreaded vector assigned to them to provide frequency-diversity at thereceiver site. The scheme uses frequency domain analysis to estimateinterference, which is used for weighting each received subcarrierbefore despreading. This results in switching off those subcarrierscontaining the interference.

Despite the suggestions in the prior art to combine certain of themultiple access protocols to improve bandwidth efficiency, there hasbeen little success in implementing such combinations. It becomes moredifficult to calculate optimum operating parameters as more protocolsare combined. The networks implementing combined multiple accessprotocols become more complex and expensive. Accordingly, theimplementation of high-bandwidth efficiency communications using acombination of multiple access protocols continues to be a challenge.

SUMMARY OF THE INVENTION

In at least one embodiment of the invention, an apparatus includes atransmitter module of a communications unit configured to transmit afirst data signal. The first data signal is allocated to a first set oforthogonal frequency division multiplexed discrete frequency tones in afirst frequency band. The transmitter module is configured to transmitthe first data signal during a first set of time intervals of a frame.The apparatus includes a receiver module of the communications unitconfigured to receive a second data signal. The second data signal isreceived using a second set of orthogonal frequency division multiplexeddiscrete frequency tones in a second frequency band. The receiver moduleis configured to receive the second data signal during a second set oftime intervals of the frame. The first and second frequency bands aredifferent.

In at least one embodiment of the invention, a method includestransmitting, by a communications unit, a first data signal, using afirst set of orthogonal frequency division multiplexed discretefrequency tones in a first frequency band of a plurality of frequencybands and during a first set of time intervals, fewer than all timeintervals, of a plurality of time intervals of a frame. The methodincludes transmitting, by the communications unit, a second data signal,using a second set of orthogonal frequency division multiplexed discretefrequency tones in the first frequency band, and during the first set oftime intervals. The first and second sets of orthogonal frequencydivision multiplexed discrete frequency tones are different.

In at least one embodiment of the invention, a method includestransmitting, by a communications unit, a first data signal, using afirst set of a plurality of sets of orthogonal frequency divisionmultiplexed discrete frequency tones in a first frequency band of aplurality of frequency bands and during a first set of time intervals ofa frame. The method includes transmitting, by the communications unit, asecond data signal, using the first set of the plurality of sets oforthogonal frequency division multiplexed discrete frequency tones inthe first frequency band, and during a second set of time intervals ofthe frame. The first and second sets of time intervals are distinct.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is an architectural diagram of the PWAN FDD system, includingremote stations communicating with a base station.

FIG. 1.1 is a diagram of PWAN Airlink RF Band Organization

FIG. 1.2 is a diagram of Physical Channels.

FIG. 1.3 is a diagram of PWAN Physical Layer Framing Structure

FIG. 1.4 is a diagram of Details of TDMA Slot Parameters

FIG. 1.5 is a diagram of A PWAN 64 kbit/s data channel

FIG. 1.6 is a diagram of Functional Block Diagram of Base transmitterfor a single traffic channel in rate 3/4 16 QAM mode.

FIG. 1.7 is a diagram of The block diagram for Base CLC/BRCtransmissions

FIG. 1.8 is a diagram of Gray-coded mapping for the QPSK modulation onthe CLC/BRCchannel.

FIG. 1.9 is a diagram of The demultiplexing of a CLC/BRC message on twoconsequent TDMA frames

FIG. 1.10 is a diagram of The Functional Block Diagram of Base receiverfor a single traffic channel in rate 3/4 16 QAM mode

FIG. 1.11 is a diagram of The Functional Block Diagram of Base receiverfor a CAC

FIG. 1.12 is a diagram of Functional Block Diagram of RU transmitter fora single traffic channel in rate 3/4 16 QAM mode

FIG. 1.13 is a diagram of The block diagram for RU CAC transmissions

FIG. 1.14 is a diagram of The demultiplexing of a CAC message on twoconsequent TDMA frames.

FIG. 1.15 is a diagram of The Functional Block Diagram of RU receiverfor a single traffic channel in rate 3/4 16 QAM mode

FIG. 1.16 is a diagram of The baseband representation of the RU CLC/BRCreceiver

FIG. 2.1 is a diagram of Functional Block Diagram of Base transmitterfor a single traffic channel in rate 3/4 16 QAM mode.

FIG. 2.2 is a diagram of The Functional Block Diagram of Base receiverfor a single traffic channel in rate 3/4 16 QAM mode

FIG. 3.1 is a diagram of Forward beam pattern and its effect on RU RSSI

FIG. 3.2 is a diagram of Forward beam pattern altered to accommodateincoming RU

FIG. 4.1 is a diagram of Processing Diagram

FIG. 4.2 is a diagram of Signals as seen at the Base station

FIG. 4.3 is a diagram of Delay Compensation in action

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is an architectural diagram of the frequency division duplex(FDD) personal wireless access network (PWAN) system, in accordance withthe invention. The system employs the method of the invention thatcombines time division duplex (TDD), frequency division duplex (FDD),time division multiple access (TDMA), orthogonal frequency divisionmultiplexing (OFDM), spatial diversity, and polarization diversity invarious unique combinations.

FIG. 1 provides an overview of how the invention combines TDD, FDD,TDMA, and OFDM to enable the base station Z to efficiently communicatewith many remote stations U, V, W, and X. The base station Z receives afirst incoming wireless signal 10 comprising a plurality of firstdiscrete frequency tones F2 that are orthogonal frequency divisionmultiplexed (OFDM) in a first frequency band from the first remotestation U during a first time division multiple access (TDMA) interval.The organization of the TDMA intervals is shown in FIG. 1.5, which isdiscussed in detail below. Then the base station Z receives a secondincoming wireless signal 12 comprising a plurality of second discretefrequency tones F4 that are orthogonal frequency division multiplexed(OFDM) in the first frequency band from a second remote station W duringthe first time division multiple access (TDMA) interval. The first andsecond stations U and W accordingly have different sets of discretefrequency tones F2 and F4, that are orthogonal frequency divisionmultiplexed.

The base station Z in FIG. 1, receives a third incoming wireless signal14 comprising a plurality of the first discrete frequency tones F2 thatare orthogonal frequency division multiplexed (OFDM) in the firstfrequency band from a third remote station V during a second timedivision multiple access (TDMA) interval. The first and second TDMAintervals are part of the same TDMA frame, as is shown in FIG. 1.5. Thefirst and third stations U and V, accordingly are time divisionmultiplexed by sharing the same set of discrete frequency tones F2 indifferent TDMA intervals.

The base station Z in FIG. 1, receives a fourth incoming wireless signal16 comprising a plurality of the second discrete frequency tones F4 thatare orthogonal frequency division multiplexed (OFDM) in the firstfrequency band from a fourth remote station X during the second timedivision multiple access (TDMA) interval. The second and fourth stationsW and X accordingly are time division multiplexed by sharing the sameset of discrete frequency tones F4 in different TDMA intervals.

The base station Z in FIG. 1, transmits the first outgoing wirelesssignal 18 comprising a plurality of third discrete frequency tones F1that are orthogonal frequency division multiplexed (OFDM) in a secondfrequency band to the first remote station U during a third timedivision multiple access (TDMA) interval. The first remote station U andthe base station Z accordingly are time division duplexed (TDD) bytransmitting their respective signals 10 and 18 at different TDMAintervals. The first, second, third, and fourth TDMA intervals occur atmutually different times, as is shown in FIG. 1.5. In addition, thefirst remote station U and the base station Z accordingly are frequencydivision duplexed (FDD) by transmitting their respective signals 10 and18 on different sets of discrete frequency tones F2 and F1 in differentfrequency bands.

The base station Z in FIG. 1, transmits the second outgoing wirelesssignal 20 comprising a plurality of fourth discrete frequency tones F3that are orthogonal frequency division multiplexed (OFDM) in the secondfrequency band to the second remote station W during the third timedivision multiple access (TDMA) interval. The second remote station Wand the base station Z accordingly are time division duplexed (TDD) bytransmitting their respective signals 12 and 20 at different TDMAintervals. In addition, the second remote station W and the base stationZ accordingly are frequency division duplexed (FDD) by transmittingtheir respective signals 12 and 20 on different sets of discretefrequency tones F4 and F3 in different frequency bands.

The base station Z in FIG. 1, transmits the third outgoing wirelesssignal 22 comprising the plurality of the third discrete frequency tonesF1 that are orthogonal frequency division multiplexed (OFDM) in thesecond frequency band to the third remote station V during a fourth timedivision multiple access (TDMA) interval. The third remote station V andthe base station Z accordingly are time division duplexed (TDD) bytransmitting their respective signals 14 and 22 at different TDMAintervals. In addition, the third remote station V and the base stationZ accordingly are frequency division duplexed (FDD) by transmittingtheir respective signals 14 and 22 on different sets of discretefrequency tones F2 and F1 in different frequency bands.

The base station Z in FIG. 1, transmits the fourth outgoing wirelesssignal 24 comprising the plurality of the fourth discrete frequencytones F3 that are orthogonal frequency division multiplexed (OFDM) inthe second frequency band to the fourth remote station X during thefourth time division multiple access (TDMA) interval. The fourth remotestation X and the base station Z accordingly are time division duplexed(TDD) by transmitting their respective signals 16 and 24 at differentTDMA intervals. In addition, the fourth remote station X and the basestation Z accordingly are frequency division duplexed (FDD) bytransmitting their respective signals 16 and 24 on different sets ofdiscrete frequency tones F4 and F3 in different frequency bands.

FIG. 1 shows another embodiment of the invention, wherein TDD, FDD,TDMA, OFDM, and space diversity are combined to enable the base stationto efficiently communicate with many remote stations. This is possiblebecause of the multiple element antenna array A, B, C, and D at the basestation Z that is controlled by despreading and spreading weights. Thespreading weights enable the base station Z to steer the signals ittransmits to remote stations U and V that are have a sufficientgeographic separation from one another. The despreading weights enablethe base station Z to steer the receive sensitivity of the base stationtoward the sources of signals transmitted by remote stations U and Vthat have a sufficient geographic separation from one another. Toillustrate the effectiveness of spatial diversity in this embodiment,remote stations U and V share the same discrete frequency tones F1 andF2 and the same TDMA interval.

The base station Z in FIG. 1, receives a first incoming wireless signal10 comprising a plurality of first discrete frequency tones F2 that areorthogonal frequency division multiplexed (OFDM) in a first frequencyband from the first remote station U at a first geographic locationduring a first time division multiple access (TDMA) interval. The basestation Z in FIG. 1, receives a second incoming wireless signal 14comprising a plurality of the first discrete frequency tones F2 that areorthogonal frequency division multiplexed (OFDM) in the first frequencyband from the second remote station V at a second geographic locationduring the same, first time division multiple access (TDMA) interval.The base station Z in FIG. 1, spatially despreads the first and secondincoming signals 10 and 14 received at the base station Z by usingspatial despreading weights. Spatial diversity is provided because thedespreading weights enable the base station Z to steer the receivesensitivity of the base station toward the first remote station U andthe second remote station V, respectively.

Later, the base station Z in FIG. 1, spatially spreads a first andsecond outgoing wireless signals 18 and 22 at the base station by usingspatial spreading weights. Then the base station Z in FIG. 1, transmitsthe first outgoing wireless signal 18 comprising a plurality of thirddiscrete frequency tones F1 that are orthogonal frequency divisionmultiplexed (OFDM) in a second frequency band to the first remotestation U at the first geographic location during a third time divisionmultiple access (TDMA) interval. The base station Z in FIG. 1, transmitsthe second outgoing wireless signal 22 comprising a plurality of thethird discrete frequency tones F1 that are orthogonal frequency divisionmultiplexed (OFDM) in the second frequency band to the second remotestation V at the second geographic location during the same, third timedivision multiple access (TDMA) interval. Spatial diversity is providedbecause the spreading weights enable the base station Z to steer thesignals it transmits to the first and second remote stations U and V,respectively.

FIG. 1 shows still another embodiment of the invention, wherein TDD,FDD, TDMA, OFDM, and polarization diversity are combined to enable thebase station Z to efficiently communicate with many remote stations U,V, W, and X. This is possible because the antenna A, B, C, or D at thebase station Z and the antennas at the remote stations U, V, W, and Xare designed to distinguish orthogonally polarized signals. Signalsexchanged between the base station Z and a first remote station U arepolarized in one direction, and signals exchanged between the basestation Z and a second remote station V are polarized in an orthogonaldirection. To illustrate the effectiveness of polarization diversity inthis embodiment, remote stations U and V share the same discretefrequency tones F1 and F2 and the same TDMA interval.

The base station Z in FIG. 1, receives a first incoming wireless signal10 polarized in a first polarization direction comprising a plurality offirst discrete frequency tones F2 that are orthogonal frequency divisionmultiplexed (OFDM) in a first frequency band from the first remotestation U during a first time division multiple access (TDMA) interval.The base station Z in FIG. 1, receives a second incoming wireless signal14 polarized in a second polarization direction comprising a pluralityof the first discrete frequency tones F2 that are orthogonal frequencydivision multiplexed (OFDM) in the first frequency band from a secondremote station V during the first time division multiple access (TDMA)interval. The base station Z in FIG. 1, distinguishes the first andsecond incoming signals 10 and 14 received at the base station bydetecting the first and second polarization directions. Polarizationdiversity is provided because signals exchanged between the base stationZ and the first remote station U are polarized in one direction, andsignals exchanged between the base station Z and the second remotestation V are polarized in an orthogonal direction.

Later, the base station Z in FIG. 1, forms a first and second outgoingwireless signals 18 and 22 at the base station by polarizing them in thefirst and second polarization directions, respectively. Then the basestation Z in FIG. 1, transmits the first outgoing wireless signal 18polarized in the first polarization direction comprising a plurality ofthird discrete frequency tones F1 that are orthogonal frequency divisionmultiplexed (OFDM) in a second frequency band to the first remotestation U at the first geographic location during a third time divisionmultiple access (TDMA) interval. Then the base station Z in FIG. 1,transmits the second outgoing wireless signal 22 polarized in the secondpolarization direction comprising a plurality of the third discretefrequency tones F1 that are orthogonal frequency division multiplexed(OFDM) in the second frequency band to the second remote station V atthe second geographic location during the third time division multipleaccess (TDMA) interval. Polarization diversity is provided becausesignals exchanged between the base station Z and the first remotestation U are polarized in one direction, and signals exchanged betweenthe base station Z and the second remote station V are polarized in anorthogonal direction.

In still a further embodiment of the invention, TDD, FDD, TDMA, OFDM,spatial diversity, and polarization diversity are combined to enable abase station Z to efficiently communicate with many remote stations U,V, W, and X. The resulting invention makes highly efficient use ofscarce bandwidth resources to provide good service to a large populationof users.

The PWAN system has a total of 3200 discrete tones (carriers) equallyspaced in 10 MHZ of available bandwidth in the range of 1850 to 1990MHZ. The spacing between the tones is 3.125 kHz. The total set of tonesare numbered consecutively form 0 to 3199 starting from the lowestfrequency tone. The tones are used to carry traffic messages andoverhead messages between the base station and the plurality of remoteunits.

In addition, the PWAN system uses overhead tones to establishsynchronization and to pass control information between the base stationand the remote units. A Common Link Channel (CLC) is used by the base totransmit control information to the Remote Units. A Common AccessChannel (CAC) is used to transmit messages from the Remote Unit to theBase. There is one grouping of tones assigned to each channel. Theseoverhead channels are used in common by all of the remote units whenthey are exchanging control messages with the base station.

Selected tones within each tone set are designated as pilots distributedthroughout the frequency band. Pilot tones carry known data patternsthat enable an accurate channel estimation. The series of pilot tones,having known amplitudes and phases, have a known level and are spacedapart by approximately 30 KHz to provide an accurate representation ofthe channel response (i.e., the amplitude and phase distortionintroduced by the communication channel characteristics) over the entiretransmission band.

Section 1 PWAN FDD Physical Layer

1.1 Overview

The PWAN FDD system uses a TDMA structure to provide various data rates,and to allow a hybrid FDD/TDD technique at the remote station (RU). FDDis used in the sense that the Base and the RU transmit and receive ontwo separate bands, and the TDD is used to indicate that for a givenconnection both the Base and the RU transmit and receive on differentTDMA slots. This has no effect on the overall system capacity, and isonly a measure for simplifying the RU design; i.e., to ensure that aduplexer is not needed at the RU.

1.2 Frequency Definitions

The total bandwidth allocation for the airlink of the PWAN Network is 10MHZ in the PCS spectrum which is in the range of 1850 to 1990 MHZ. Thetotal bandwidth is divided into two 5 MHZ bands called the Lower RF Bandand the Upper RF Band. The separation between the lowest frequency inthe Lower RF Band and the lowest frequency in the Upper RF Band (DF) is80 MHZ. The base frequency (fbase) for the PWAN Network is defined asthe lowest frequency of the Lower RF Band which depends on the specificPCS frequency band. As shown in FIG. 1.1, the PWAN frequency assignmentconsists of a lower and an upper frequency band.

There are a total of 1600 tones (carriers) equally spaced in each of the5 MHZ of available bandwidth. The spacing between the tones is 3.125kHz. The total set of tones are numbered consecutively from 0 to 3199starting from the lowest frequency tone. Ti is the frequency of the ithtone:

$T_{i} = \left\{ \begin{matrix}{f_{base} + \frac{\Delta\; f}{2} + {{i \cdot \Delta}\; f}} & {0 \leq i \leq 1599} \\{f_{base} + {\Delta\; F} + \frac{\Delta\; f}{2} + {{i \cdot \Delta}\; f}} & {1600 \leq i \leq 3199}\end{matrix} \right.$where fbase is the base frequency, Df is 3.125 kHz, and DF is 80 MHZ.Equivalently, the relationship may be expressed as:

$T_{i} = \left\{ \begin{matrix}{f_{base} + {{\left( {i + \frac{1}{2}} \right) \cdot 3.125}\mspace{14mu}{kHz}}} & {0 \leq i \leq 1599} \\{f_{base} + 80000 + {{\left( {i + \frac{1}{2}} \right) \cdot 3.125}\mspace{14mu}{kHz}}} & {1600 \leq i \leq 3199}\end{matrix} \right.$

The set of 3200 tones is the Tone Space. The tones in the Tone Space areused for two functions: transmission of bearer data, and transmission ofoverhead data. The tones used for the transmission of bearer data arethe Bearer Tones, and the tones dedicated to pilot channels are theOverhead Tones.

Bearer Tones

The bearer tones are divided into 160 Physical Channels which consist of80 Forward Physical Channels (FPC) and 80 reverse Physical Channels(RPC). Some of these channels are unavailable because they must be usedas a guardband between PWAN and other services in the neighboring bands.Each of the Physical channels contains 18 Tones as shown in FIG. 1.2.The mapping of tones into the ith FPCi, and the ith RPCi is shown inTable 1.1 and Table 1.2 respectively.

Overhead Tones

The overhead tones are used for the following channels:

-   -   Forward Control Channel: FCC    -   Reverse Control Channel: RCC        These channels may use any set of the 160 overhead tones. The        following equation shows the mapping of overhead tones:        FCC(i)=T _(10i) 0≦i≦159        RCC(i)=T _(1600-10i) 0≦i≦159        1.3 Timing and Framing Definitions

The framing structure is shown in FIG. 1.3. The smallest unit of timeshown in this figure is a TDMA Slot. 8 TDMA slots constitute a TDMAframe. 16 TDMA frames make a multiframe, and 32 multiframes make asuperframe. Frame synchronization is performed at the superframe level.The multiframe boundary is determined from the superframe boundary.

As shown in FIG. 1.4, in every TDMA slot, there is a transmission burstand a guard time. Data is transmitted in each burst using multipletones. The burst duration is Tburst. A guard period of duration Tguardis inserted after each burst. Table 1.3 shows the values of the TDMAslot parameters.

1.4 Bearer Channel Definitions

A PWAN bearer channel uses a single physical channel with 18 tonesseparated by 3.125 kHz. The bandwidth occupancy of a bearer channel istherefore 56.25 kHz. Bearer channels may be used to carry traffic orcontrol (access and broadcast) information.

The PWAN traffic channels may carry between 16 kbit/s to 64 kbit/s ofinformation depending on the number of TDMA slots assigned to them. A 16kbit/s PWAN traffic channel uses one TDMA slot per TDMA frame, a 32kbit/s channel uses 2 TDMA slots per frame, and a 64 kbit/s channel uses4 TMDA slots per TDMA frame, as shown in FIG. 1.5. FIG. 1.5 assumes thatthere is no space division multiple access. However, it may be possibleto support more than one user on a given TDMA slot, if the users aregeographically separated and the transceiver can take advantage of thatseparation to form spatial beams.

1.5 Transmission Formats

1.5.1 Traffic Channel Modulation Modes

In order to increase the overall capacity of the system, and to ensureviable deployment of the system in various interference levels,propagation environments, and possible required transmission ranges,PWAN may use various coded modulation schemes (rates). Under benignchannel conditions, an efficient high rate code may be used. Ifconditions get worse, a low rate, coded modulation scheme is used. Thisis especially important considering the high availability requirementsfor a wireless local loop system.

As an example, we consider the use of a bandwidth efficient rate 3/4 16QAM scheme. Lower rate codes can also be used.

1.5.2 Base Transmission Format

1.5.2.1 Base Transmitter Functional Block Diagram

1.5.2.1.1 Traffic Channels (an example)

The Base transmits information to multiple RUs in its cell. This sectiondescribes the transmission formats for a 16 kbit/s to 64 kbit/s trafficchannel, together with a 1 kbit/s to 4 kbit/s Link Control Channel (LCC)from the Base to a single RU. The 16 kbit/s link is achieved byassigning one TDMA slot per TDMA frame. The TDMA frame is 3 ms long,thus the effective data rate is 16 kbit/s times the number of TDMA slotsper TDMA frame. For higher data rates, the process described in thissection is repeated in every applicable TDMA slot. For example, for the64 kbit/s link, 4 TDMA slots per frame need to be assigned; in whichcase, the process described here is repeated 4 times within a given TDMAframe. The block diagram for Base transmitter in FIG. 1.6 shows theprocessing of data for one TDMA slot.

The Binary Source delivers 48 bits of data in one TDMA slot. The bit tooctal conversion block converts the binary sequence into a sequence of3-bit symbols. The symbol sequence is converted into a 16 elementvector. One symbol from the Link Control Channel (LCC) is added to forma 17 element vector.

The vector is trellis encoded. The trellis encoding starts with the mostsignificant symbol (first element of the vector) and is continuedsequentially until the last element of the vector (the LCC symbol). Theoutput of the trellis encoder is a 17 element vector where each elementis a signal within the set of 16QAM constellation signals.

A known pilot symbol is added to form an 18 element vector, with thepilot as the first element of this vector.

The resulting vector is to be transmitted over 8 different antennas. Theelements of the vector are weighted according to the antenna elementthrough which they are transmitted. The description of how these weightscan be derived is found in Section 2.

The 18 symbols destined for each antenna are then placed in the inverseDFT frequency bins (corresponding to the physical channel) where theyare converted into the time domain. The symbols are mapped into tones onthe ith forward physical channel FPCi. The mapping of the common linkchannel (CLC)/broadcast channel (BRC) symbols into tones is shown inTable 1-4. The digital samples are converted to analog, RF converted andsent to the corresponding antenna element (0 to 7) for transmission overthe air.

This process is repeated from the start for the next 48 bits of binarydata transmitted in the next applicable TDMA slot. FIG. 1.6 is aFunctional Block Diagram of Base transmitter for a single trafficchannel in rate 3/4 16 QAM mode.

1.5.2.1.2 CLC/BRC Channels

The block diagram for the CLC/BRC transmissions is shown in FIG. 1.7.The generation of CLC/BRC information is represented by a binary sourcethat generates 72 bits of data for every CLC/BRC transmission. The 72bit sequence is RS encoded using a shortened Reed Solomon RS (63, 35)code to generate a 40 RS symbol sequence (or equivalently a 240 bitsequence).

The 240 bit sequence is then quadrature phase shift key (QPSK) modulatedwhere every two bits are mapped onto a constellation point according tothe Gray mapping shown in FIG. 1.8.

The output of the QPSK modulator is a 120 symbol sequence (S0-S119). TheQPSK symbols are interleaved with 24 pilot symbols (P0-P23), where forevery 5 data symbols, a pilot is inserted. This results in a 144 symbolsequence. The sequence is then time demultiplexed into 8, 18 elementvectors for transmission over 8 TDMA slots (in two TDMA frames) as shownin FIG. 1.9.

A given 18 element vector is transmitted over 8 different antennas. Theelements of the vector are weighted according to the antenna elementthrough which they are transmitted. The description of how these weightscan be derived is found in Section 2.

The 18 symbols destined for each antenna are then placed in the inverseDFT frequency bins (corresponding to the physical channel) where theyare converted into the time domain. The digital samples are converted toanalog, RF converted and sent to the corresponding antenna element (0 to7) for transmission over the air. Table 1.4 shows a mapping of symbolsonto tones for CLC/BRC transmissions on the ith physical channel.

1.5.2.2 Base Receiver Functional Block Diagram

1.5.2.2.1 Traffic Channels (Example)

FIG. 1.10 shows the block diagram of the Base receiver for a trafficchannel. During a given TDMA slot, and on a given physical channel, theBase receives signals on all its 8 antennas. The signals aredown-converted, digitally sampled, and transformed back into frequencydomain using Discrete Fourier Transform (DFT). For a particular trafficchannel, the appropriate tones are selected using a demultiplexer. Thetones from all the antennas are then sent to a despreader. Thedespreader weights all the tones from a given antenna by a given weightwhich can be calculated as described in Section 2, and then adds all thetones from different antennas (addition of 8, 18 element vectors). Theresulting 18 element vector is then sent to an equalizer where eachelement of the vector is multiplied by a phase correction factor, andthe pilot symbol is stripped off the sequence. The remaining 17 symbolsare sent to the trellis decoder which delivers 16 symbols (48 bits) oftraffic data, and 1 symbol (3 bits) of LCC data.

This process is repeated from the start for the next 48 bits of binarydata transmitted in the next applicable TDMA slot. FIG. 1.10 is aFunctional Block Diagram of Base receiver for a single traffic channelin rate 3/4 16 QAM mode.

1.5.2.2.2 Common Access Channels (CACs)

FIG. 1.11 shows the block diagram of the Base receiver for a CAC. Duringa given TDMA slot, and on a given physical channel, the Base receivessignals on all its 8 antennas. The signals are down-converted, digitallysampled, and transformed back into frequency domain using DiscreteFourier Transform (DFT). For a particular CAC channel, the appropriatetones are selected using a demultiplexer. The tones from all theantennas are then sent to a despreader. The despreader weights all thetones from a given antenna by a given weight which can be calculated asdescribed in Section 2, and then adds all the tones from differentantennas (addition of 8, 18 element vectors). The resulting 18 tones aresent to an equalizer where each tone is multiplied by a phase correctionfactor, and the 9 pilot symbols are stripped off the sequence. The 9element vector is then QPSK demodulated. Since each element of thevector is a QPSK symbol representing two bits of information, thedemodulator outputs 18 bits of information.

The time multiplexer collects the symbols received in 8 consecutive TDMAslots to form 144 bits of RS encoded information hence forming a 24 RSsymbol block (every RS symbol is 6 bits long). The RS block is thendecoded to produce 12 RS symbols or 72 bits of the original CACinformation transmitted from the RU. FIG. 1.11 is a Functional BlockDiagram of Base receiver for a CAC.

1.5.2.3 RU Transmitter Functional Block Diagram

1.5.2.3.1 Traffic Channels

The RU transmits information to the single Base in its cell. Thissection describes the transmission formats for a 16 kbit/s to 64 kbit/straffic channel, together with a 1 kbit/s to 4 kbit/s Link ControlChannel (LCC) from an RU to its Base. The 16 kbit/s link is achieved byassigning one TDMA slot per frame. For higher data rates, the processdescribed in this section is repeated in every applicable TDMA slot. Forexample, for the 64 kbit/s link, 4 TDMA slots per frame need to beassigned.

The block diagram for the RU transmitter in FIG. 1.12 shows theprocessing of data for one TDMA slot.

The Binary Source delivers 48 bits of data in one TDMA slot. The bit tooctal conversion block converts the binary sequence into a sequence of3-bit symbols. The symbol sequence is converted into a 16 elementvector. One symbol from the Link Control Channel (LCC) is then added toform a 17 element vector.

The vector is trellis encoded. The output of the trellis encoder isanother 17 element vector where each element is a signal within the setof 16QAM constellation signals.

A known pilot symbol is then added to form an 18 element vector. The 18elements are placed in the inverse DFT frequency bins (corresponding tothe physical channel) where they are converted into the time domain. Thedigital samples are converted to analog, RF converted and sent toantenna for transmission over the air.

This process is repeated from the start for the next 48 bits of binarydata transmitted in the next applicable TDMA slot. FIG. 1.12 is aFunctional Block Diagram of RU transmitter for a single traffic channelin rate 3/4 16 QAM mode.

1.5.2.3.2 Common Access Channels (CACs)

The block diagram for the RU CAC transmissions is shown in FIG. 1.13.The generation of CAC information is represented by a binary source thatgenerates 72 bits of data for every CAC transmission. The 72 bitsequence is RS encoded using a shortened Reed Solomon RS (63, 35) codeto generate a 24 RS symbol sequence (or equivalently a 114 bitsequence).

The 114 bit sequence is QPSK modulated where every two bits are mappedonto a constellation point according to Gray mapping. The output of theQPSK modulator is therefore a 72 symbol sequence (S0-S71). The QPSKsymbols are interleaved with 72 known pilot symbols (P0-P71), where forevery data symbol, a pilot is inserted. This results in a 144 symbolsequence. The sequence is time demultiplexed into 8, 18 element vectorsfor transmission over 8 TDMA slots (in two TDMA frames) as shown in FIG.1.14. Table 1.5 Mapping of symbols onto tones for CAC transmissions onthe ith reverse physical channel

During each TDMA slot, the 18 symbols are placed in the DFT frequencybins (corresponding to the physical channel) where they are convertedinto the time domain. The digital samples are converted to analog, RFconverted and sent to the antenna for transmission over the air. FIG.1.14 shows the demultiplexing of a CAC message on two consequent TDMAframes.

1.5.2.4 RU Receiver Functional Block Diagram

1.5.2.4.1 Traffic Channels

FIG. 1.15 shows the block diagram of the RU receiver. During a givenTDMA slot, and on a given physical channel, the RU receives a signal onits antenna. The signals are down-converted, digitally sampled, andtransformed back into frequency domain using Discrete Fourier Transform(DFT). For a particular traffic channel, the appropriate tones areselected using a demultiplexer. The 18 tones are sent to an equalizerwhere each tone is multiplied by a phase correction factor, and thepilot symbol is stripped off the sequence. The remaining 17 symbols aresent to the trellis decoder which delivers 16 symbols (48 bits) oftraffic data, and 1 symbol (3 bits) of LCC data.

FIG. 1.15 shows the Functional Block Diagram of RU receiver for a singletraffic channel in rate 3/4 16 QAM mode.

1.5.2.4.2 Common Link and Broadcast Channels (CLC/BRCs)

FIG. 1.16 is a block diagram representation of baseband processing inthe CLC/BRC receiver. During a given TDMA slot, on a given physicalchannels used for CLC/BRC transmission, the RU receives a time domainsignal through its antenna. The signal is down-converted, digitallysampled, and transformed back into frequency domain using DiscreteFourier Transform (DFT). The appropriate tones for the CLC/BRC areselected using a demultiplexer. The 18 tones are sent to an equalizerwhere each tone is multiplied by a phase correction factor, and the 3pilot symbols are then stripped off. The remaining 15 elements are thenQPSK demodulated. Hence, the demodulator outputs 30 bits of information.

The time multiplexer collects the data in 8 consecutive TDMA slots toform 240 bits of RS encoded information hence forming a 40 RS symbolblock (every RS symbol is 6 bits long). The RS block is then decoded toproduce 12 RS symbols or 72 bits of the original CLC/BRC informationtransmitted from the Base.

Section 2 PWAN FDD Spatial Processing

Introduction

Spatial processing is incorporated into the PWAN physical layer toprovide enhanced capacity and an improved grade of service. These areachieved through spatial isolation for frequency reuse and through thesuppression of co-channel interferers. In a frequency division duplex(FDD) system where the transmit and receive bands are widely separatedin frequency, reciprocity in the channel is not achievable. Therefore,different beamforming strategies are needed on the forward and reversechannels.

This section specifically describes those functions required toimplement spatial processing at the PWAN Base station. Spatialprocessing at the Remote Unit is optional and offers a potential meansof gaining spatial degrees of freedom for further increases in capacity.

The primary functions present in any beamforming system will bedescribed for both the forward and reverse links. They include theapplication of the beamforming weights, weight computation, adaptation,and incorporation of the reference pilots.

2.1 Narrowband Assumption

It is first important to address one of the primary assumptions inherentin any beamforming system; whether it is narrowband or broadband infrequency. For the PWAN system, it is assumed that all beamforming isnarrowband. It is necessary to define the system as narrowband at theoutset to be sure that the frequency responses at different arrayelements match closely and that received spatial samples aresufficiently correlated from one end of the array to the next.

This assumption can be examined analytically in the context of theobservation time-bandwidth product for the Base station antennaaperture. It can also be tested through observation of the mismatchpresent in the beam pattern across the frequency band of interest.

The observation-time interval for an antenna aperture is defined as thetime required for a plane waveform to travel completely across theantenna aperture. This is a function of the signal angle-of-arrival. Theobservation time-bandwidth product (TBWP) is the product of theobservation interval and the signal bandwidth. For an array to beconsidered narrowband, the TBWP should be much less than 1 for allangles of arrival.

A quick calculation of the TBWP for a linear aperture with 8 elementuniform spacing should give a bound on this TBWP for PWAN, since thiswould be the limiting case on the observation interval for a uniformlyspaced array. Equation 2.1 and Equation 2.2 describe this calculation,where Td is the observation time interval, BW is the signal bandwidth(112.5 kHz), N is the number of array elements (8), and c is the speedof light.

$\quad\begin{matrix}\begin{matrix}{{Td} = {\left( {{Element}\mspace{14mu}{Spacing}*{\sin\left( {{angle}\text{-}{of}\text{-}{arrival}} \right)}} \right)/}} \\{\left. c \right)*\left( {N - 1} \right)} \\{= {{\left( {{2.63\; e} - 10} \right)*\left( {8 - 1} \right)} = {{1.84\; e} - 09}}}\end{matrix} & \left( {{Eq}\mspace{14mu} 2.1} \right)\end{matrix}$TBWP=1.84e-09*BW=1.84e-09*112.5e03=2.07e-04TBWP=2.07e-04<<1  (Eq 2.2)

This calculation was done for the maximum angle-of-arrival off boresight(at endfire) of 90 degrees for the maximum observation interval. As canbe seen from equation 1.2, the TBWP is much less than one for themaximum delay (all other angles would have smaller TBWP) so thenarrowband assumption holds.

This assumption was also investigated for the linear array throughsimulations with the result that mismatch in the beampatterns over thisbandwidth had negligible mean-square error (MSE); again validating thenarrowband assumption.

2.2 Far Field Assumption

Another important assumption inherent in the PWAN beamforming system isthe far-field assumption. This states that all beamforming functions aredesigned for waveforms received from the far-field (>˜4 meters) asopposed to the near field (<˜4 meters). This allows the designer totreat any propagating waveform impinging on the antenna aperture as aplanar wavefront, thus implying that propagation of the signal betweentwo antenna elements can be characterized as pure delay. The signal isassumed to have equal intensity at any point on the planar wavefront.

2.3 Forward Channel

On the forward link, from the Base to the RU, beamforming is employed toprovide isolation between spatially separated RUs. At transmission fromthe Base, beamforming is based on direction-of-arrival estimates derivedfrom received RU transmissions. At the RU receiver, spatial isolation isprovided by data combining at the antenna aperture itself for acontinuous fixed beampattern.

2.3.1 Base Transmitter

A functional representation of a single traffic channel for the Basetransmitter is presented in FIG. 2.1. This section will address thosecomponents of the Base transmitter that directly implement the spatialprocessing functions; the Spreader block shaded in FIG. 2.1 and theantenna array. FIG. 2.1 is a Functional Block Diagram of Basetransmitter for a single traffic channel in rate 3/4 16 QAM mode.

2.3.1.1 Antenna Array

The antenna array for the Base transmitter consists of N sensors thattransmit spatially weighted signals from each array element. This arrayis configured in a hemispherical geometry with either uniform ornon-uniform element spacing.

2.3.1.2 Weight Application

Beamforming in the forward direction is achieved by applying a complexweight matrix, W, composed of weight vectors corresponding to each user,such that the transmitted signal is reinforced in the direction of thedesired RU(s) and attenuated for all other transmit directions. Thisweight application is described in the following equation.X=WY  (Eq 2.3)Equation 2.3 represents a matrix multiplication of the baseband tonedata, Y, by the weight matrix, W, to produce the tone data to bemodulated and transmitted, X.

2.3.1.3 Weight Derivation

The Spreader block within the Base transmitter provides the matrix ofspatial weights, W, for application to the tone data to be transmitted.This matrix may be derived by one of several methods; fromdata-independent fixed beam weights derived from the spatial separationof the antenna array aperture, by real-time, adaptive, computation ofstatistically optimum beam weights derived from second order statisticsof the data received over the antenna array aperture, or by receivingeither type of weight through an interface from another Base subsystem.

Each method of weight derivation implies certain advantages with regardto spatial isolation, co-channel interference rejection, and systemprocessing complexity.

Data independent fixed beam weights place the least real-timecomputational burden on the baseband subsystem, since these weights canbe pre-calculated and tabularized for look-up on a per call basis atreal-time. Data-independent weights provide spatial isolation betweendownlink transmissions for spatial division multiple access (SDMA) but,will not steer nulls for direct suppression of co-channel interferers.However, suppression of these interferers is inherent in the spatialisolation provided by the mainlobe-to-sidelobe ratio, but the advantages(spatial resolution and increased attenuation) of adaptively steeringnulls directly at a co-channel interferers are not available.

Statistically optimum beamforming weights provide direct suppression ofco-channel interferers, but require increased processing power and causeincreased latency due to convergence times in the weight calculation. Itshould also be noted that in FDD systems, direction of arrival (DOA)estimation will not provide the transmit beamformer with accurateestimates of multipath components due to the independence of channeleffects over the wide frequency separation of the forward and reversebands. This implies that the transmit beamformer will only be able tonull fixed line of sight (LOS) transmissions from interfering RUs.Another significant benefit to this approach, however, is the increasedresolution of the mainbeam, thus providing enhanced spatial resolutionover data-independent weights.

2.3.1.4 Weight Update

The weight matrix, W, must be periodically updated. This update isrequired to track changes in the forward link channel as well asindividual links continuously being torn down and set up over thatchannel. For data independent weights, channel tracking does not applyand the weight update is based on a per call basis. For statisticallyoptimum weights, the update interval is based on the rate of adaptationderived from the channel statistics (time-bandwidth product and channelstationarity.)

2.3.1.5 Reference Signal

The Base will transmit reference link maintenance pilots (LMPs) toprovide a phase reference for coherent demodulation and an estimate ofthe desired signal for beamform error estimation. Reference signals aretransmitted on both the forward link and on the reverse link. Thisallows the Base to make a measurement of mean-square error (MSE) for usein deriving an optimal beamform solution on the reverse link withoutrelying on direction-of-arrival estimation.

2.3.1.6 Direction-of-Arrival Estimation

A priori knowledge of the angle-of-arrival for RU transmissions isrequired to steer transmit beams at the desired signals on acall-by-call basis. This information must be gathered through DOAestimation techniques.

2.3.2 RU Receiver

RU baseband spatial processing techniques perform spatial processing atthe array aperture to form a continuous beam.

2.4 Reverse Channel

On the reverse link, from the RU to the Base, beamforming is employed toprovide isolation between spatially separated RUs and to providesuppression of co-channel interferers. On this link, no a prioriknowledge of the angle-of-arrival of RU transmissions is required but,rather, reference signal adaptive beamforming is employed.

2.4.1 Base Receiver

A functional representation of a single traffic channel for the Basereceiver is presented in FIG. 2.2. This section addresses thosecomponents of the Base receiver that directly implement the spatialprocessing functions; the Despreader block shaded in FIG. 2.2 and theantenna array. FIG. 2.2 shows the Functional Block Diagram of Basereceiver for a single traffic channel in rate 3/4 16 QAM mode.

2.4.1.1 Antenna Array

The antenna array for the Base receiver consists of N sensors thatsample waveforms impinging on the array aperture at each array element.This array is configured in a hemispherical geometry with either uniformor non-uniform element spacing.

2.4.1.2 Weight Application

Spatial processing in the reverse direction is achieved by applying acomplex weight matrix, W, composed of weight vectors corresponding toeach user, such that the received signal is reinforced in the directionof the desired RU and attenuated in all other directions. This weightapplication is described in the following equation.Y=WX  (Eq 2.4)Equation 2.4 represents a matrix multiplication of the baseband tonedata, X, by the weight matrix, W, to produce the despread tone data,Y.

2.4.1.3 Weight Derivation

The Despreader block within the Base receiver provides the matrix ofspatial weights, W, for application to the received data. This matrixmay be derived by one of several methods; from data-independent fixedbeam weights derived from the spatial separation of the antenna arrayaperture, by real-time, adaptive, computation of statistically optimumbeam weights derived from second-order statistics received over theantenna array aperture, or by receiving either type of weight through aninterface from another Base subsystem.

Each method of weight derivation implies certain advantages with regardto spatial isolation, co-channel interference rejection, and systemprocessing complexity.

Data independent fixed beam weights place the least real-timecomputational burden on the baseband subsystem, since these weights canbe pre-calculated and tabularized for look-up on a per call basis atreal-time. Data-independent weights provide spatial isolation betweenuplink transmissions for spatial division multiple access (SDMA) but, donot steer nulls for direct suppression of co-channel interferers.However, suppression of these interferers is inherent in the spatialisolation provided by the mainlobe-to-sidelobe ratio, but the advantages(spatial resolution and increased attenuation) of adaptively steeringnulls directly at a co-channel interferers is not available.

Statistically optimum beamforming weights provide direct suppression ofco-channel interferers, but require increased processing power and causeincreased latency due to convergence times in the weight calculation.

2.4.1.4 Weight Update

The weight matrix, W, must be periodically updated. This update isrequired to track changes in the reverse link channel as well asindividual links continuously being torn down and set up over thechannel. For data independent weights, channel tracking does not applyand the weight update is based on a per call basis. For statisticallyoptimum weights, the update interval is based on the rate of adaptationderived from the channel statistics (time-bandwidth product and channelstationarity.)

2.4.1.5 Direction-of-Arrival Estimation

On the reverse link, direction-of-arrival estimation is performed on thereceived data set for application on the forward link. This informationallows the Base transmit beamformer to steer beams at the intended RUand, for statistically optimum weights, nulls at interfering RUs. For awireless local loop system such as PWAN, no tracking of theangle-of-arrival estimate is necessary since the source RUs are fixed inspace. As mentioned previously, these angle-of-arrival estimates havethe disadvantage of being independent across PCS bands so that they willnot track multipath effects.

2.4.1.6 Reference Pilots

The Base will transmit reference link maintenance pilots (LMPs) toprovide a phase reference for coherent demodulation and an estimate ofthe desired signal for beamform error estimation. Reference signals aretransmitted on both the forward link and on the reverse link. Thisallows the Base to make a measurement of mean-square error (MSE) for usein deriving an optimal beamform solution on the reverse link withoutrelying on direction-of-arrival estimation.

2.4.2 RU Transmitter

RU baseband spatial processing techniques RU perform spatial processingat the array aperture to form a continuous beam.

Section 3 PWAN Channel Allocation

Introduction

When a traffic channel is to be established for an RU the base mustallocate a channel on which the RU can meet the required grade ofservice.

3.1 RU Capability

The channel allocation algorithm needs to know information concerningthe capabilities of each active and new RU. If future equipments operateover different IF bandwidths then the channel allocation algorithm needsto know what each equipment can support.

In the first generation of PWAN the RU's support a 1 MHz operatingbandwidth. This allows operation over any of 16 channels. When an RU isidentified by its RUID the base searches a data base containing therequired information about the RU for the channel allocation algorithme.g. frequency (IF), bandwidth (BW), number of bearer (B) channelssupported, type(s) of voice coding supported, etc.

3.2 Direction of Arrival (DOA)

Since the system is dependent on SDMA for increasing capacity, a veryimportant parameter is the DOA for each RU. The channel allocationalgorithm needs to know the DOA of every RU involved in an active calland the DOAs of any new RUs.

Initially a channel allocation algorithm could be devised which simplymaximizes the separation of the DOA between a new RU and the otheractive RU's on some number of available clusters. However, as the numberof users on the system increases, there needs to be more informationincorporated into the channel selection than just DOA.

There are several candidate algorithms for DOA estimation:

-   -   coherent signal-subspace (CSS) with spatial interpolation    -   SS-DOA    -   MUSIC    -   ESPRIT        3.3 Channel Measurements

In order for the best channel to be chosen, the RUs must makemeasurements on some number of channels and report the results to thebase station for use in selecting the best channel for an RU when a linkis established. These measurements include RSSI and SINR. Table 3.1shows a gross look at how received signal strength indicator (RSSI) andsignal to interference ratio (SINR) information could be used to assignchannels to incoming RUs.

It is clear that the lower the RSSI on a channel the better a candidateit would be since there is little energy directed at that RU on thatchannel by any base. However an RU could measure energy from a forwardantenna pattern sidelobe as shown in FIG. 3.1. From FIG. 3.1 it is seenthat the incoming RU could be accommodated if the serving base alteredits beam pattern for the established RU to steer a null at the incomingRU. Likewise the beam pattern for the incoming RU would have a nullsteered at the established RU. This situation is shown in FIG. 3.2. FIG.3.1 shows Forward beam pattern and its effect on RU RSSI. FIG. 3.2 showsForward beam pattern altered to accommodate incoming RU.

With only RSSI information it is impossible to distinguish between thebeam sidelobe of the serving base and interference from surroundingbases. To help decide between intercell and intracell energy the SINRmeasurements are used. A low SINR value indicates high levels of noiseand interference on the channel. A high SINR value indicates a clearsignal from the serving base. So for the situation shown in FIG. 3.1 theRU would report significant RSSI with fairly high SINR. If the DOA ofthe incoming RU was far enough away from the established RU then thereis enough information to know that the forward beam pattern can besquinted to accommodate the incoming RU on that channel as shown in FIG.3.2.

As a first cut it seems that the three pieces of data can be combinedinto a channel candidacy assessment factor (CAF). The three desirablesituations are: large separation in DOA, small RSSI, and high SINR. Soan equation to quantify the candidacy of channel n isCAF(n)=fDOA(n)+fRSSI(n)−fSINR(n)  (Eq 3.1)fDOA(n)=a1(180−min(|DOAe(k)−DOAi|)) for all k  (Eq 3.2)fRSSI(n)=a2(133+RSSI(n))  (Eq 3.3)fSINR(n)=a3(SINRRU(n)+SINRBase(n))  (Eq 3.4)

A lower value of CAF indicates a better candidacy for that channel. Anideal channel would have a CAF of 0. In Equation 3.2 through Equation3.4 a1, a2, and a3 are scale factors for the three terms. The first termof Equation 3.1 assesses the DOA information. The maximum separationpossible is 180 degrees. So a larger difference in DOA will cause thefirst term to be smaller. The second term of Equation 3.1 assesses theRSSI measurement. The noise floor of the receiver is −133 dBm. This isthe ideal measurement indicating no activity on that channel, so anyvalue greater than −133 dBm biases the CAF away from ideal. The thirdterm of Equation 3.1 includes the effects of channel SINR. A largervalue of SINRRU gives a better CAF since it means the energy seen by theRU is from the serving base. Likewise a larger SINRBase means there isless interference from RUs in other cells on that channel.

EXAMPLE

a1=a2=1, a3=1/2

An RU requests a traffic channel from its serving cell. It reports thefollowing measurements as part of traffic establishment: RSSI(1)=−95dBm, SINRRU(1)=9.3 dBRSSI(2)=−95 dBm, SINRRU(2)=4.5 dB.

The serving base measures the incoming RU's DOA as 42 degrees. There isan RU on channel 1 at 127 degrees, and one on channel 2 at 133 degrees.Also, SINRBase(1)=12 dB, and SINRBase(2)=13 dB.

$\begin{matrix}{{{CAF}(1)} = {\left( {180 - {{127 - 42}}} \right) + \left( {133 - 95} \right) - {0.5\left( {9.3 + 12} \right)}}} \\{= 122.35}\end{matrix}$ $\begin{matrix}{\;{{{CAF}(2)} = {\left( {180 - {{133 - 42}}} \right) + \left( {133 - 95} \right) - {0.5\left( {4.5 + 13} \right)}}}} \\{= 118.25}\end{matrix}$

The RU measured the same RSSI on both channels. The DOA of the existingRU on channel 1 was slightly better than the DOA of the existing RU onchannel 2. The base's SINR measurement for channel 1 was slightly betterthan the measurement for channel 2. The measurement that made thebiggest difference in this case was the RU SINR.

In order to effectively measure SINR, the RUs and base must have a senseof whether or not a signal belongs to a given cell. Without any suchmechanisms a SINR measurement would be the same for a signal of a givenRSSI level from the serving base as it would be for a signal from aneighboring base with the same RSSI value. A way to differentiatesignals in different cells is to encode the reference pilots on thetraffic channels with different phases or sequence of phases which wouldbe derived from the Base Station Offset Code.

3.4 Procedure

-   -   1. In idle mode each RU measures the RSSI and SINR of each        channel it could potentially operate on and orders them from        subjective best to subjective worst.    -   2. There is a parameter, meas_rpts, which is sent on the        broadcast channel of each base stating how many channel        measurements an RU will send to the base when a traffic        connection is to be established.    -   3. When a traffic connection is to be established the RU sends        the best meas_rpts channel measurements to the base on the SCAC        channel.    -   4. The base uses the channel measurements sent by the RU to        compute a CAF for each of the candidate channels in the set sent        by the RU.    -   5. If one or more of the channels in the set which was sent by        the RU produces an acceptable CAF then the channel with the best        CAF is chosen.    -   6. If none of the channels in the set which was sent by the RU        produces an acceptable CAF then the base requests the next best        set of meas_rpts measurements from the RU.    -   7. The base repeats steps 4 through 6 until either an acceptable        channel is found and is then used or the list of candidate        channels is exhausted at which point the call is blocked.

Section 4 PWAN Synchronization

Introduction

The RU synchronization and Base delay compensation algorithms arediscussed. Both algorithms aim to achieve synchronization in either time(Base delay compensation), or in time and frequency (RUsynchronization).

4.1 RU Synchronization

When the remote unit (RU) is initialized and begins receivingtransmissions from its Base, the time of arrival of the waveform isunknown. Also, the RU Signal Pilots (RSPs) will not likely be within theprescribed FFT bins because the Base oscillator and the RU oscillatorare operating at slightly different frequencies. The purpose of thesynchronization algorithm is to align the RU processing window, orreceive gate, with the waveform, and to adjust the RU referenceoscillator (VCXO) to operate at the same frequency as the Baseoscillator.

Synchronization is presented as a two-step process. First, acquisitionof the synchronization waveform in both time and frequency must occur.The RU receive gate is adjusted to contain most of the signal energy,and the RU VCXO is adjusted to eliminate most of the RU-Base frequencydisparity. Driving the residual frequency offset to zero and maintainingan average frequency offset of zero requires a robust method offrequency estimation, continuously running during RU operation. Once thefrequency error is eliminated, the RU is said to be frequency-locked tothe base. Maintaining the zero frequency error is the function of thefrequency-tracking step, which runs continuously in the background. Thephase-locked loop (PLL) is capable of tracking time-varying phaseimmersed in noise, and is thus an effective frequency estimator fortracking RU-Base frequency errors. In fact, the PLL is theimplementation of the optimal; i.e., maximum likelihood, carrier phaseestimator.

The only requirement for the algorithm is that the system is based onorthogonal frequency division multiplexing (OFDM), with even spacingbetween tones.

4.1.1 Time Required for Synchronization

RU synchronization is performed at initialization of the RU, or wheneversynchronization is lost. The time requirements for achieving initialsynchronization are not as critical as for the case when the RU has toreestablish frequency lock after sleep mode. RU battery life is thecritical issue in keeping resync time at a minimum.

Time and frequency bandwidth are the resources that determine how longit takes to achieve frequency lock. Channel effects and noise must beaveraged out in the estimation of time delay and frequency offsets; onecan either average over time, or over frequency, to mitigate theseeffects. A balance must be struck between use of the available bandwidthand the time constraints determined by the system requirements.

4.1.2 RU Synchronization Implementation

4.1.2.1 Synchronization Pilots

The proposed synchronization algorithm does not assume a particularmodel for the data channel configuration. That is, no particular tonemapping of the pilot waveforms, or RU signal pilots (RSPs), is assumed.The RSPs can comprise the overhead tones of a data channel, be in aseparate synchronization channel, or constitute part of a messageframing structure.

4.1.2.2 Functional Description

The ultimate objective of synchronization is to achieve time andfrequency lock for demodulation of data. Precise alignment of the RUreceive time gate with the data burst, and frequency lock of the RUoscillator with the Base is required for orthogonality of the FFT bindata, and hence, for reliable demodulation. Synchronization relies upona multiple step procedure in which coarse adjustments are made in timeand frequency, then fine adjustments are made in time and frequency tosystem specifications. The steps, labeled coarse time alignment, coarsefrequency alignment, fine time alignment, and frequency tracking, areoutlined below.

-   -   1. Coarse Time Alignment. The waveform must be within the        receive gate for determination of the RSPs in frequency. Coarse        time alignment is achieved with a filter matched to the        frequency-offset waveform.    -   2. Coarse Frequency Alignment. Outputs from a bank of matched        filters in the frequency domain yield a coarse estimate of the        frequency offset. The RU VCXO is adjusted to bring the RU        oscillator within a specified frequency tolerance of the Base        oscillator.    -   3. Frequency Track. A phase-locked loop (PLL) drives the        residual frequency offset to zero and continuously adjusts the        RU VCXO to keep the average frequency difference at zero.    -   4. Fine Time Alignment. This aligns the waveform with the RU        receive gate to within the final required accuracy.

4.1.2.3 Processing Description

A high level block diagram of the processing steps is given in FIG. 4.1.FIG. 4.1 shows the methods used to accomplish the objective of eachprocessing step. Except for the frequency tracking stage, matchedfiltering constitutes the primary tool for time and frequencyacquisition. Realization of the matched filter differs in each step, butthe concept is identical. Coarse time alignment uses a filter impulseresponse that is matched to the frequency-offset waveform. Coarsefrequency alignment uses a bank of filters to estimate the frequencyoffset. The final time alignment step uses a single filter that is tunedto the exact specified frequency.

4.2 Delay Compensation

When a RU is installed, it needs to know when to transmit its signals inrelation to the signals received from the Base station so that itssignal will arrive at the base station at the same time as the signalsfrom the other RUs. The Base station measures the difference between theexpected time of arrival and the actual time of arrival of the RUsignals. It then transmits this measurement information to the RU so itcan advance or delay the time that it sends signals to the Base station.

FIG. 4.2 shows the signals that appear at the Base station. The Basestation expects to see the signals from the RU arrive 55 us after ittransmits its last burst. FIG. 4.3 shows the signals that appear at theBase and the RU. Before the RU is compensated, the signals it transmitsarrive at the Base at a different time to the signals transmitted by theother RUs. The Base measures the delay and transmits the measurement tothe RU. The RU then adjusts the time of transmission to compensate forthe delay.

Delay compensation can be performed upon installation and also at everycall setup. The Delay Compensation calculation routine examines theaverage signal power in the signals used for the delay calculations andif they are above a certain threshold then a delay compensationmeasurement is made. FIG. 4.3 shows the Delay Compensation in action.

4.2.1 Algorithm Description

Delay compensation relies on measuring the phase of pilot tones calleddelay compensation pilots (DCPs). The RU transmits the DCPs to the basestation with each DCP having the same phase shift. If the RU has beencompensated properly the DCP tones arrive at the base station in phasewith each other. If the signal from the RU is delayed then each of theDCP tones experiences a phase shift, which is proportional to the DCPfrequency. The Base measures the phase of each DCP and uses linearregression to fit the phases to a straight line. The slope of this lineis proportional to the delay. A slope of zero indicates that no delaycompensation is needed, whereas a nonzero slope means that the signal isarriving too early (or late) and the RU needs to delay (or advance)transmission of its signal.

Multipath effects and noise will corrupt the phase measurements. Thiscan be mitigated by averaging the phase measurements over frequency(over DCPs) and over time (over successive data bursts).

Section 5 Diversity

Introduction

Diversity is a communication receiver technique that exploits the randomnature of radio propagation by finding highly uncorrelated signal pathsfor communication. Diversity decisions are made by the receiver. If oneradio path undergoes a deep fade, another independent path may have astrong signal. By having more than one path to select from, both theinstantaneous and average signal to noise ratios at the receiver may beimproved.

In Space diversity, multiple base station or remote station receivingantennas are used to provide diversity reception.

In polarization diversity, horizontal and vertical polarization pathsbetween a remote station and a base station are uncorrelated. Thedecorrelation for the signals in each polarization is caused by multiplereflections in the channel between the remote station antennas.Depending on the characteristics of the link between a given remote andits base station.

Frequency diversity transmits information on more than one carrierfrequency.

Time diversity repeatedly transmits information at time spacings thatexceed the coherence time of the channel, so that multiple repetitionsof the signal will be received with independent fading conditions.

There are four categories of diversity reception methods:

-   -   1. Selection diversity    -   2. Feedback diversity    -   3. Maximal ratio combining    -   4. Equal gain diversity

In Selection diversity m demodulators are used to provide m diversitybranches. The receiver branch having the highest instantaneous SNR isconnected to the demodulator.

In Feedback or Scanning Diversity, the m signals are scanned in a fixedsequence until one is found to be above a predetermined threshold.

In Maximal Ratio Combining, the signals from all of the m branches areweighted according to their individual signal voltage to noise powerrations and then summed.

In Equal Gain Combining, the branch weights are all set to unity but thesignals from each branch are co-phased to provide equal gain combiningdiversity.

Although frequency diversity is used to mitigate fading, it is not thesole means. In an FDD-based system in which the coherence bandwidth mayexceed the available bandwidth, the effectiveness of frequency diversityis not sufficient to avoid fading. Receiving on orthogonal polarizationcomponents is used as a complementary means to combat fading.Polarization diversity is used in the PWAN system.

5.1 Frequency Diversity

The maximum frequency separation possible in an 8×2 (spatial×frequency)implementation for the 5 MHZ band is 2.5 MHZ. The frequency separationmust exceed the channel coherence bandwidth to minimize the likelihoodthat both carriers are simultaneously faded. The coherence bandwidthscorresponding to correlations of 90% and 50% between frequencies aretypically used for roughly characterizing the channel. An estimate ofthe coherence bandwidth at the 0.90 correlation level is given byequation:

$B_{c} \approx \frac{1}{50\;\sigma_{\tau}}$where is the rms delay spread. When the coherence bandwidth is definedfor 0.50, correlation between frequency components, becomes

$B_{c} \approx \frac{1}{5\;\sigma_{\tau}}$

Table 5.1 lists as a function of correlation level and upper and lowerbounds on rms delay spreads measured at PCS bands including both line ofsight and non-line of sight paths.

At the 50% correlation level, the upper bound of exceeds the 2.5 MHZfrequency spacing available for frequency diversity in the PWAN FDDsystem. The data presented here is not intended as the definitivemeasure of the coherence bandwidths expected in the PWAN deployment.Rather, it is intended to show that given the uncertainties in theenvironment, the coherence bandwidth can easily exceed the availablesystem bandwidth for frequency diversity. Given this, another mechanism,such as polarization, must be considered for diversity.

5.2 Polarization Diversity

5.2.1 Introduction

Polarization diversity exploits the tendency of multipath to spillenergy from a transmitted polarization component into the orthogonalpolarization component. For example, a transmitter may emit a verticallypolarized component, but the receiver would receive energy in both thevertical and horizontal polarization components. If fading affects onecomponent and not the other, then the signal could still be received ina polarization diverse system.

5.2.2 Results

Polarization diversity for 900 MHZ and 1800 MHZ systems can provide adiversity gain comparable to horizontal spatial diversity gain.Polarization diversity is used in conjunction with or in lieu offrequency diversity. Some conclusions are:

-   -   The correlation between fading of the horizontal and vertical        polarization components in multipath is significantly less than        0.70 95% of the time. Correlation values are less than 0.10.    -   Polarization diversity receive systems can provide performance        improvement over a single linear polarization channel in a        strong multipath environment. The performance is comparable with        that provided by a horizontal spatial diversity system.    -   Diversity performance is enhanced when the transmitting antenna        strongly excites a horizontal component in addition to the        vertical component. This occurs with a slant 45 degree linear        polarization or circular polarization. In this case, the average        received signal experiences a loss of less than 1 dB compared        with the spatial diversity system.    -   Maximal ratio combining of the polarization branches is        generally assumed in the papers. This provides the best        performance compared to selection diversity and equal gain        combining. Compared with selection diversity, maximal ratio        combining can provide as much as a 3 dB gain benefit in        mitigating multipath effects.    -   In a multipath environment, the typical cross-polarization level        is about −10 dB relative to the polarization level of the        transmitting antenna.        5.3 The Polarization Diverse System

The orthogonal polarization components may be sufficiently decorrelatedto provide protection against multipath fading. (However, the highdegree of cross-polarization that makes diversity possible works againstpolarization as an interference suppression mechanism.) Theimplementation scenario for polarization diversity in the PWAN system isthe following:

-   -   Polarization diversity reception at both the Base and RU    -   Transmitting with either a slant-45 degree linear polarization,        or with circular polarization    -   Receiving with either a dual slant 45 degree linear        polarization, or with vertical (V) and horizontal (H) components    -   Combining the polarization branches with either maximal ratio        combining or equal gain combining A trade-off between the        optimality of the maximal ratio combining and the        implementational ease of equal gain combining will determine the        best solution.

The resulting invention makes highly efficient use of scarce bandwidthresources to provide good service to a large population of users.

Although the preferred embodiments of the invention have been describedin detail above, it will be apparent to those of ordinary skill in theart that obvious modifications may be made to the invention withoutdeparting from its spirit or essence. Consequently, the precedingdescription should be taken as illustrative and not restrictive, and thescope of the invention should be determined in view of the followingclaims.

What is claimed is:
 1. An apparatus comprising: a transmitter module ofa communications unit configured to transmit a first data signal, thefirst data signal being allocated to a first set of orthogonal frequencydivision multiplexed discrete frequency tones in a first frequency band,and the transmitter module being configured to transmit the first datasignal during a first set of time intervals of a frame; and a receivermodule of the communications unit configured to receive a second datasignal, the second data signal being received using a second set oforthogonal frequency division multiplexed discrete frequency tones in asecond frequency band and the receiver module being configured toreceive the second data signal during a second set of time intervals ofthe frame, the first and second frequency bands being different, whereinthe transmitter module comprises: an encoder configured to encode thefirst data signal to generate an encoded first data signal; a tonemapper module configured to map the encoded first data signal to thefirst set of orthogonal frequency division multiplexed discretefrequency tones in a frequency domain signal; an inverse discreteFourier transform module configured to transform the frequency domainsignal into a digital time domain signal; a digital-to-analog converterconfigured to convert the digital time domain signal into an analogbaseband signal; a radio frequency transmitter configured to convert theanalog baseband signal into a radio frequency signal; and an antennaconfigured to transmit the radio frequency signal.
 2. The apparatus, asrecited in claim 1, wherein the first and second sets of time intervalsare different, and the first and second sets of orthogonal frequencydivision multiplexed discrete frequency tones are different.
 3. Theapparatus, as recited in claim 1, wherein the receiver module comprises:an antenna configured to receive a radio frequency signal; a radiofrequency receiver configured to convert the radio frequency signal to abaseband signal; an analog-to-digital converter configured to digitallysample the baseband signal to generate a digital version of the basebandsignal; a discrete Fourier transform block configured to transform thedigital version of the baseband signal from a time domain signal into afrequency domain signal; a tone select module configured to select thesecond set of orthogonal frequency division multiplexed discretefrequency tones from the frequency domain signal; and a decoderconfigured to recover the second data signal from a signal at leastpartially based on the second set of orthogonal frequency divisionmultiplexed discrete frequency tones.
 4. A method comprising:transmitting, by a communications unit, a first data signal, using afirst set of orthogonal frequency division multiplexed discretefrequency tones in a first frequency band of a plurality of frequencybands and during a first set of time intervals, fewer than all timeintervals, of a plurality of time intervals of a frame; transmitting, bythe communications unit, a second data signal, using a second set oforthogonal frequency division multiplexed discrete frequency tones inthe first frequency band, and during the first set of time intervals;and receiving a third data signal, the third data signal being allocatedto a third set of orthogonal frequency division multiplexed discretefrequency tones in a second frequency band of the plurality of frequencybands different from the first frequency band, and during a second setof time intervals, fewer than all time intervals, of the plurality oftime intervals of the frame, wherein the first and second sets oforthogonal frequency division multiplexed discrete frequency tones aredifferent.
 5. The method, as recited in claim 4, wherein transmittingthe first data signal comprises: encoding the first data signal togenerate an encoded first data signal; mapping the encoded first datasignal to the first set of tones in a frequency domain signal;transforming the frequency domain signal into a digital time domainsignal; converting the digital time domain signal into an analogbaseband signal; and converting the analog baseband signal into a radiofrequency signal; and transmitting the radio frequency signal using anantenna.
 6. The method, as recited in claim 4, wherein the first andsecond sets of time intervals are different and the first and third setsof orthogonal frequency division multiplexed discrete frequency tonesare different.
 7. The method, as recited in claim 4, further comprising:receiving a fourth data signal, the fourth data signal being allocatedto a fourth set of orthogonal frequency division multiplexed discretefrequency tones in the second frequency band, and during a third set oftime intervals, fewer than all time intervals, of the plurality of timeintervals of the frame.
 8. The method, as recited in claim 7, whereinthe second and third sets of time intervals are different and the thirdand fourth sets of the plurality of sets of orthogonal frequencydivision multiplexed discrete frequency tones are the same.
 9. Themethod, as recited in claim 7, wherein the second and third sets of timeintervals are different and the third and fourth sets of the pluralityof sets of orthogonal frequency division multiplexed discrete frequencytones are different.
 10. The method, as recited in claim 4, furthercomprising: receiving a radio frequency signal using an antenna;converting the radio frequency signal from radio frequencies into abaseband signal; sampling the baseband signal to generate a digitalbaseband signal; transforming the digital baseband signal from a timedomain signal into a frequency domain signal; selecting the third set oforthogonal frequency division multiplexed discrete frequency tones fromthe frequency domain signal; and recovering the third data signal from asignal at least partially based on the third set of tones.
 11. A methodcomprising: transmitting, by a communications unit, a first data signal,using a first set of a plurality of sets of orthogonal frequencydivision multiplexed discrete frequency tones in a first frequency bandof a plurality of frequency bands and during a first set of timeintervals of a frame; transmitting, by the communications unit, a seconddata signal, using the first set of the plurality of sets of orthogonalfrequency division multiplexed discrete frequency tones in the firstfrequency band, and during a second set of time intervals of the frame,wherein the first and second sets of time intervals are distinct; andreceiving a third data signal, the third data signal being allocated toa second set of the plurality of sets of orthogonal frequency divisionmultiplexed discrete frequency tones in a second frequency band of theplurality of frequency bands different from the first frequency band,and during a third set of time intervals, fewer than all time intervals,of the plurality of time intervals of the frame.
 12. The method, asrecited in claim 11, wherein transmitting the first data signalcomprises: encoding the first data signal to generate an encoded firstdata signal; mapping the encoded first data signal to the first set oftones in a frequency domain signal; transforming the frequency domainsignal into a digital time domain signal; converting the digital timedomain signal into an analog baseband signal; converting the analogbaseband signal into a radio frequency signal; and transmitting theradio frequency signal using an antenna.
 13. The method, as recited inclaim 11, wherein the first and third sets of time intervals aredifferent and the first and second sets of discrete frequency tones aredifferent.
 14. The method, as recited in claim 11, further comprising:receiving a fourth data signal, the fourth data signal being allocatedto a third set of the plurality of sets of orthogonal frequency divisionmultiplexed discrete frequency tones in the second frequency band, andduring a fourth set of time intervals, fewer than all time intervals, ofthe plurality of time intervals of the frame.
 15. The method, as recitedin claim 14, wherein the third and fourth sets of time intervals aredifferent and the second and third sets of the plurality of sets oforthogonal frequency division multiplexed discrete frequency tones arethe same.
 16. The method, as recited in claim 14, wherein the third andfourth sets of time intervals are different and the second and thirdsets of the plurality of sets of orthogonal frequency divisionmultiplexed discrete frequency tones are different.
 17. The method, asrecited in claim 11, further comprising: receiving a radio frequencysignal using a first antenna; converting the radio frequency signal intoa baseband signal; sampling the baseband signal to generate a digitalbaseband signal; transforming the digital baseband signal from a timedomain signal into a frequency domain signal; selecting the second setof tones from the frequency domain signal; and recovering the third datasignal from a signal at least partially based on the third set of tones.